This invention relates generally to operational amplifiers, and more particularly to a low-power Class A bipolar operational amplifiers utilizing unbalanced device sizes to compensate for input-stage offset voltages and suitable for use in low-power medical devices such as hearing aids and the like.
Typically, a Class A operational amplifier includes a balanced input circuit comprised of first and second differential input transistors that receive inverting and non-inverting inputs at their respective base electrodes. The input-stage provides an output signal at the collector of the second input transistor. Associated with the first and second input transistors are first and second load transistors, the base electrodes of which are coupled to the collector of the first input transistor. The emitter electrodes of the input transistors are coupled to a source of supply voltage (e.g. ground) via a first current source (Ib1).
The output of the input-stage is applied to an output stage that in turn has an output terminal for coupling to a load. The output stage must be capable of sinking relatively large currents associated with the load and therefore generally includes a relatively large current source (Ib2).
To be suitable for use in certain medical devices such as hearing aids, wristwatch-type pulse monitors, implantable medical devices, and the like, an operational amplifier must be capable of operating at low-power with low supply voltages (e.g. 1.5 volts). Thus, it is desired that Ib1 be relatively small, in particular less than Ib2, which in turn must be large enough to drive the load. Unfortunately, this difference creates non-random base current imbalances which in turn cause the currents in the collectors of the first and second input transistors to be perturbed to different degrees creating an undesirable input offset voltage.
In the past, the problem of input offset voltage in bipolar operational amplifiers has been addressed in several ways. For example, an emitter follower may be used as a buffer between the input and output stages. Alternatively, a higher current may be utilized in the front-end differential pair, and yet another known approach involves the use of base current cancellation techniques. Unfortunately, these solutions are not completely compatible with low-power designs.
In view of the foregoing, it should be appreciated that it would be desirable to provide a lower power bipolar operational amplifier which exhibits substantially reduced input offset voltages for use in low-power medical devices. Additional desirable features will become apparent to one skilled in the art from the foregoing background of the invention and following detailed description of a preferred exemplary embodiment and appended claims.
In accordance with an aspect of the present invention, there is provided a low-power operational amplifier comprising a differential input-stage and an output stage. The differential input-stage includes first and second differentially coupled input transistors each having base, emitter, and collector terminals. A first current mirror circuit is coupled to the first and second input transistors and produces a first current which perturbs the current flowing through the first input transistor. The output stage is coupled to the differential input-stage and to the current mirror circuit and produces a second larger current which perturbs the current flowing through the second input transistor. The ratio of the emitter areas of the first and second input transistors are selected to substantially eliminate offset voltage caused by the difference between the first and second perturbing currents.